While everything you say is valid, they are reasons she is a virgin at conception and at birth.
They are not compelling reasons why she would inexorably and necessarily be a virgin afterwards.
Your extrapolation leads to conjecture that is untenebale - you can't point to one direct scriptural reference that Mary always remained a virgin after the birth. No one can.
Mary asks the angel Gabriel how can she conceive since she does not know man.
If she was to marry Joseph and have relations, wouldn't her question be nonsensical?
The exchange strongly implies Mary's perpetual virginity.
Martin Luther as well as other reformers believed in her perpetual virginity. Besides the constant witness of the Church, Roman and Greek, as well as even some of the early Reformers, how can one living 2000 years later be so sure of himself that he would wish to publicly cast doubt on what had been a constant, universal belief throughout Christendom for 1500 years?